Refractive index optical biosensor can be used in applications such as drinking water contaminations monitor, early detection of infectious viral diseases and real-time study of molecular interaction in chemical and biological processes. Generally, sensors with a high sensitivity have a high sensitivity to temperature. Thus, temperature of a testing condition can greatly influence the measured results of the sensors. However, conventional approach of using polymer negative thermal optic (TO) cladding to overcome this problem cannot be utilized in biosensors.
In the market of optical refractive index biosensor, one widely used method is through surface plasmon resonance (SPR). Surface Plasmon is a charge density wave occurring at the interface between a metal and a dielectric. It can be stimulated when a beam of light is incident on the interface of a metal layer and the biochemical target with a certain angle between the light and the surface. This angle can be shifted according to the change of the refractive index of the biochemical. By monitoring the incident angle or the intensity of the reflected light, this change of the refractive index can be achieved. The simplicity of experimental set-up and the reasonable sensitivities obtained using surface plasmon made this technology a practical and commonly used method in the application of biosensor. The SPR biosensor can provide the following of biospecific interactions in real time instead of the determining of the concentration of the target chemical. However, the size and cost of the instrument are still very large which limited the application of the SPR biosensor.
Silicon photonics can offer a platform for the chip-size integration of these big components with more functions and lower cost. For example, the incident light prism coupling method in SPR biosensor can be replaced by etching grating coupler on the surface of the silicon chip for fiber coupling. The complex optical routes set up in SPR biosensor can be replaced by standard plane waveguide routes on Silicon-On-Insulator and the big-size output signals collection part in the SPR biosensor can also be replaced by integrating germanium photo detectors on the same chip with that of biosensor. The advantages of shrinking the biosensor to chip size can include automation of the analysis, shorter response time, reduced manual sampling handling and low cost. Utilizing silicon photonics platform, volume refractive index and surface mass density detection limit of biosensor made on array of silicon nitride slot waveguide micro-ring resonators may reach 5.0e-6 refractive index units (RIUs) and 0.9 pg/mm2 respectively. However, the impact of temperature is more serious in silicon waveguide-based biosensors because of the large thermal-optical effect of the silicon material. Micro-ring resonators may help to increase the sensitive of the sensor but the measure error coming from environment temperature may become larger. Thermal drift may be compensated using on-chip referencing, external thermal modulator or Peltier heat pump in the platform. However, the complexity and cost may be increased at the same time.